In the design of high density, CMOS large scale integrated circuits, it is necessary to provide effective insulation between circuit components in and on a silicon substrate. A convenient and effective means of providing such insulation is to grow a thick field oxide where desired to segregate and protect circuit components. The technique of oxidation of the silicon to form such isolated islands is recognized as being very advantageous.
It is common to utilize silicon nitride as an oxidation mask for the local oxidation of silicon to form insulated islands of silicon. It is recognized, however, that silicon nitride deposited directly onto the surface of the wafer causes defects to be formed in the silicon due to intrinsic stress in the silicon nitride film. The difference in thermal expansion between the silicon and silicon nitride adds significantly to the defect-producing stresses.
The problem of stress-induced defects in silicon caused by the above-described process has been solved by the use of what has been termed a "pad oxide" layer. This is a thin layer of silicon dioxide, i.e. about 500 angstroms or less, grown on the silicon surface prior to deposition of the silicon nitride. The use of a layer of pad oxide relieves the stress generated by the silicon nitride and, therefore, substantially eliminates defects in the silicon. However, although the use of a pad oxide relieves one problem, it has been found to create another.
It has been found that the use of a layer of pad oxide gives rise to what is commonly referred to as a "bird's beak" structure. In the isolation oxidation process using a pad oxide and a patterned layer of silicon nitride as a mask, the pad oxide causes enhanced lateral oxidation of the silicon under the silicon dioxide layer. The laterally forming oxide is thickest at the silicon nitride edge and it tapers as it penetrates under the edge of the silicon nitride. Lateral oxide penetration of from 0.6 to 1.5 micrometers is common. The penetration of laterial oxidation, in cross sectional profile, generally resembles a bird's head and beak. In accordance with this invention, a method has been found that significantly reduces both the aforementioned stress problems and the "bird's beak" formation.